1. Field of the Invention
The present invention relates to a driving circuit for a non-volatile memory device.
2. Description of the Related Art
Non-volatile memory devices (for example, flash memories) are commonly used in several applications when data must be preserved even if a power supply is off (such as in Personal Computers, cellular phones, digital still cameras, or automotive devices).
The flash memory is a particular type of E2PROM, wherein each memory cell consists of a single transistor (typically a floating gate MOS transistor). The cells of the flash memory are programmed by injecting electric charge into their floating gates; conversely, the cells are erased by discharging their floating gates. The electric charge in the floating gate of each cell modifies its threshold voltage, so as to define different logic values.
Generally, the flash memory includes one or more sectors, each one consisting of a matrix of memory cells organized into a plurality of rows and columns. The memory cells of each row are associated with a wordline, whereas the memory cells of each column are associated with a bitline.
Operation on the flash memory requires the application of different (positive or negative) bias voltages to the wordline. For this purpose, logic signals used to select a wordline must be translated into the desired bias voltage. A circuit known in the art for selecting and driving a wordline consists of two lever shifters and a selector. Each level shifter (supplied between a positive bias voltage and a reference voltage) converts a corresponding logic signal into an operative signal; when the logic signal has the value 1, the level shifter boosts up the operative signal to the positive bias voltage. The selector consists of a complementary half-bridge, which is supplied between one of the operative signals and a negative bias voltage. The selector transfers the operative signal or the negative bias voltage to the corresponding wordline, according to the other operative signal.
An intrinsic limitation of the driving circuit described above is that it does not support a negative bias voltage lower than a typical threshold voltage of the transistors used to implement the selector (such as −1.2 V). This is a drawback because the threshold voltage of memory cells that have been over-erased (depleted cells) could be lower than the negative bias voltage. When the voltage applied to the depleted cells is higher than their threshold voltage, the depleted cells are conductive, even if they are not selected. A resulting leakage current flowing in the corresponding bitlines could bring about a malfunctioning of the flash memory.